Power electronics is used in wide range of applications for controlling and converting of electric power. E.g. adjustable speed electric motor drives, consisting of a frequency converter controlled AC motor, are used widely in many industrial applications ranging from fractional kW to several MW power levels. Modern power electronic appliances are based on rapid semiconductor devices, e.g. insulated gate bipolar transistors (IGBT), used as switches in various power conversion topologies. It is always beneficial to have as low as possible internal power losses in a device, which is the reason for preferring fast components in power electronics.
The fast switching of a power component means fast voltage slope across it, which in turn gives rise to a current pulse through stray capacitances which in some measure always exist between circuits at different potentials. Most severe current of this kind is the current between an electric power distribution grid and the ground, excited by a power electronic appliance connected to the public network, due to that the high frequency ground current may be a source of interference to other electric devices connected to the same grid. This is why the ground current level of devices connected to the public network is regulated by international standards, e.g. IEC61800-3.
A normal way for limiting the ground current is to connect the power electronic appliance via a so-called common mode filter to the power supply network. This kind of a filter appears as a high impedance for common mode currents (cophasal currents in all network phases) but as a low impedance for differential mode currents (normal phase currents having sum value of 0).
Due to the formation mechanism of the ground current, its magnitude is normally the higher the higher is the switching frequency of the power components. Thus one possible method to limit ground currents is to limit the switching frequency of the power electronic devices.
In the complex impedance network of an electric installation, consisting e.g. of a supplying cable, a driving electric appliance (e.g. a frequency converter), a load cable and a load (e.g. an electric motor), there may be several frequencies where a part of the system is in resonance, i.e. frequencies where the local effective sum impedance has a very low value, comprising most of resistive components. At these frequencies the common mode current may be very high, causing e.g. overheating in the components at the current path.